mips,s390,sh,sparc: gup: Work around the "COW can break either way" issue

In Linux 4.14 and 4.19 these architectures still have their own
implementations of get_user_pages_fast().  These also need to force
the write flag on when taking the fast path.

Fixes: 407faed9 ("gup: document and work around "COW can break either way" issue")
Fixes: 5e240297 ("gup: document and work around "COW can break either way" issue")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>